Recently, increased concern over environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a major cause of air pollution. Although nickel metal hydride-based secondary batteries have mostly been used as power sources of such EVs, HEVs, and the like, a great deal of studies into use of lithium secondary batteries having high energy density, high discharge voltage, long cycle lifespan, and low self discharge rate is now extensively implemented and some thereof are commercially available.
In conventional lithium secondary batteries, a carbonaceous material is usually used as an anode active material and use of lithium metal, sulfur compounds, and the like is also considered. In addition, lithium-containing cobalt oxide (LiCoO2) is commonly used as a cathode active material, and lithium-containing manganese oxides such as LiMnO2 having a layered structure and LiMn2O4 having a spinel structure and lithium-containing nickel oxides such as LiNiO2 are also used.
Among these cathode active materials, LiCoO2 with long cycle lifespan and high charge-discharge efficiency is the most commonly used material. However, LiCoO2 entails problems such as low structural stability and high costs for cobalt as a raw material due to limited availability of cobalt, in turn reducing price competitiveness. Accordingly, there are restrictions on use of LiCoO2 in large quantities in EV applications.
Meanwhile, although LiNiO2-based cathode active materials are relatively cheap and embody high discharge capacity, they exhibit rapid phase transition in a crystal structure depending upon capacity variation accompanied by charge-discharge cycle and, when exposed to air and/or moisture, encounter sharp reduction in safety.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 have merits of excellent thermal stability and low price but entail disadvantages such as low capacity, short cycle lifespan, and poor properties at high temperature.
Among these, LiMn2O4 with a spinel structure shows relatively uniform potential in the 4V region (3.7 to 4.3V) and the 3V region (2.7 to 3.1V). However, it is known that cycle lifespan and storage properties of the above oxide are significantly deteriorated in the 3V region, thus causing difficulty in use thereof. This is because the above oxide is present in a single cubic phase in the 4V region due to phase transition based on Jahn-Teller distortion and is converted into a complex phase including two phases of the cubic phase and the tetragonal phase in the 3V region, and manganese is eluted into an electrolyte.
In addition, if spinel LiMn2O4 is synthesized using a general method such as solid state reaction, (111) planes with a lower surface energy are predominantly formed. The (111) planes have relatively low lithium ion diffusivity in comparison with other planes.
For such reasons, when a spinel lithium manganese oxide is utilized in the 3V region, real capacity of the oxide is generally lower than a theoretical capacity of the same and C-rate properties are relatively low.
Therefore, it is known that utilization of spinel lithium manganese oxides in the 3V region becomes very difficult. Some studies have reported that cycle lifespan may be increased in the 3V region by formation of a tetragonal phase and S-doping. However, such improvement is insignificant or exact reasons thereof have not been found yet.
Regarding utilization of lithium manganese oxides in the 3V region, some studies have reported a technique of increasing cycle lifespan in the 3V region by mixing a spinel lithium manganese oxide with carbon through milling. However, the present inventors have found that this technique cannot attain desired improvement in charge-discharge characteristics in the 3V region.
Therefore, there is a need to develop a technique of simply manufacturing a spinel lithium manganese oxide having high lithium ion diffusivity, high capacity, and long lifespan in the 3V region.